Fig. 7. A collision occurs when the SCS-generated antidromic AP and the SSEP-generated orthodromic AP meet at the same point while they are traveling in opposite directions. When this occurs no AP reaches the cortex to generate a response. In some very rare situations the stimuli may be sufficiently out of phase to allow an SSEP AP to pass undisturbed. However, the large difference between the SCS rate and the SSEP stimulation rate makes this condition highly unlikely. AP: action potentials, SCS: spinal cord stimulation, SSEP: standard somatosensory evoked potential.